An additional benefit of website accessibility is an improved performance in search engines. The more accessible it is to search engines, the more accurately they can predict what the site's about, and the higher your site will appear in the rankings.
Not all of the accessibility guidelines will help with your search engine rankings, but there are certainly numerous areas of overlap:
Screen readers, used by many visually impaired web users to surf the web, can't understand images. As such, to ensure accessibility an alternative description needs to be assigned to every image and the screen reader will read out this alternative, or ALT, description:
<img src="filename.gif" alt="image description goes here" />
Like screen readers, search engines can't understand images either and won't take any meaning from them. Many search engines can now index ALT text though, so by assigning ALT text search engines will be able to understand all your images.
Text embedded in images appears pixelated, blurry and often impossible to read for users utilising screen magnifiers. From an accessibility point of view this should therefore be avoided.
Search engines equally can't read text embedded in images. Well, you can just give the image some ALT text, right? Unfortunately, there's strong evidence to suggest search engines assign less importance to ALT text than they do to regular text. Why? Spammers. So many webmasters have been stuffing their ALT tags full of keywords and not using them to describe the image. Search engines have cottoned on to this form of spamming (as they eventually do every form of spamming) and have taken appropriate action.
Visually impaired web users can scan web pages by tabbing from link to link and listening to the content of the link text. As such, the link text in an accessible website must always be descriptive of its destination.
Search engines place a lot of importance on link text too. They assume that link text will be descriptive of its destination and as such examine link text for all links pointing to any page. If all the links pointing to a page about widgets say ‘click here’, search engines can't gain any information about that page without visiting it. If on the other hand, all the links say, ‘widgets’ then search engines can easily guess what that page is about.
One of the best examples of this in action is for the search term, ‘miserable failure’. So many people have linked to George Bush's bio using this phrase as the link text, that now when miserable failure is searched for in Google, George Bush's bio appears top of the search rankings!
<a href> links pointing at them.
Hearing impaired users obviously require written equivalents for audio content to be able to access it. Search engines too can't access this medium, but transcripts provide them with a large amount of text for them to index.
Site maps can be a useful tool for visually impaired users as they provide a straightforward list of links to the main pages on the site, without any of the fluff in between.
When we arrive at web pages the first thing that appears, and the first thing that visually impaired users hear, is the page title. This latter group of web users don't have the privilege of being able to quickly scan the page to see if it contains the information they're after, so it's essential that the page title effectively describes the page content.
If you know anything about search engine optimisation you'll know that the page title is the most important attribute on the page. If it adequately describes the content of that page then search engines will be able to more accurately guess what that page is about.
Visually impaired web users can scan web pages by tabbing from heading to heading, in addition to tabbing from link to link (see 3. above). As such, it's important for accessibility to make sure that headings are correctly marked up by using
Search engines assume that the text contained in heading tags is more important than the rest of the document text, as headings describe the content immediately below them. Search engines assign the most importance to
<h2>, and so on. Make sure you use the heading tags properly and don't abuse them, as the more text you have contained in heading tags, for example, the less importance search engines assign to them.
Screen readers can more effectively work through the HTML code of CSS-based sites as there's a greater ratio of content to code. Websites using CSS for layout can also be made accessible to in-car browsers, WebTV and PDAs. Don't underestimate the importance of this - in 2008 alone there'll be an estimated 58 million PDAs sold worldwide (source: eTForecast).
Search engines also prefer CSS-based sites and are likely to score them higher in the search rankings because:
With all this overlap between web accessibility and search engine optimisation there's no excuses for not implementing basic accessibility on to your website. It'll give you a higher search engine ranking and therefore more site visitors.